516 heads are 84ccs from the factory. Small exhaust valve, but that can be fixed. 915s are 79ccs. Also that single plane manifold with log exhaust manifolds aren't helping.
Ok, maybe I was thinkin about the 915s. Still though, the 516s can achieve some quench with the right piston. The bore size is a popular one, so that's a good thing. Still might have to be a custom pin height though.
